1. Field of the Invention
The present invention relates generally to calorimetric devices and, more specifically, to a method and apparatus for modulating the flux of combustibles reacting with oxygen impinging on a calorimetric gas sensor.
2. Description of the Related Art
Federal and state regulations require motor vehicles to have on-board diagnostics (OBD II) to monitor the performance of emission related components. One of the OBD II functions is to monitor the hydrocarbon (HC) conversion efficiency of a three way catalyst (TWC). Currently, this function is accomplished by using a dual oxygen sensor method. By comparing the outputs of two exhaust gas oxygen sensors, one placed upstream and the other downstream of the catalytic converter, a measure of the oxygen storage capacity is obtained, which then must be related to the hydrocarbon conversion efficiency. Although this method is presently used in production vehicles, it suffers from the disadvantage of not being sufficiently robust. The reason is that the oxygen storage capacity of the TWC has a weak relationship to the catalytic converter HC efficiency which is the desired information. Furthermore, the applicability of the dual oxygen sensor method is even more questionable for ultra low emission vehicles (ULEV) where a) a major contribution to the HC emissions comes during cold start when the HEGO sensor is not operational and b) a few percent decrease in the HC efficiency of the TWC can cause emissions above the standards. Thus, there is a need in the art for a method for monitoring the HC efficiency of the catalytic convertor directly.
One method for monitoring the HC efficiency of the catalytic convertor is to use a HC sensor to measure directly tailpipe HC emissions. One example of a HC sensor is a calorimetric gas sensor as disclosed in U.S. Pat. No. 5,451,371 to Zanini-Fisher et-al. Because of generally low concentrations of tailpipe HC in the ULEV, HC sensors must have a very low concentration detection limit and high stability. The stability of the calorimetric gas sensor is affected by zero-offset drifts. Also, when the calorimetric gas sensor is placed in the exhaust gas, the signal noise increases because the temperature fluctuations in the exhaust environment are not completely compensated by the differential nature of the sensor. Therefore, there is a need in the art to provide a method for improving the accuracy and detection limit of a calorimetric gas sensor.